1. Field of the Invention
This invention relates to a process for producing iron-ore oxidized pellets from magnetite concentrate.
2. Description of the Prior Art
It is known that the production of iron can be increased by using the so-called pelletizing technique, i.e., a process for producing pellets from iron-ore powder, for use in blast furnaces. In general, magnetite or mixtures containing magnetite will comprise over 50% of the ore used for this purpose.
Various iron-ore pellet heating and firing processes are known including the use of grate kilns, travelling grates, shaft furnaces, and circular grates. The former two are the most widely accepted at the present time. FIG. 1 shows the usual heating cycle patterns used in the production of pellets from the magnetite concentrate when using the grate kiln or grate type process. In the ordinary grate kiln type process, water and a suitable amount of a binder are first added to the starting concentrate, then the concentrate is granulated in a granulator to a grain size of 8 to 20 mm in diameter. The granules are then placed into a grate kiln for drying, and preliminary firing at a temperature of about 1000.degree.C (This preliminary firing will be referred to as "preheating" hereinafter.). The granules are then fired at a higher temperature to obtain hardened pellet products. The reason for using the preheating process is that magnetite contained in the starting material is oxidized according to the chemical reaction of 4Fe.sub.3 O.sub.4 + O.sub.2 .fwdarw.6Fe.sub.2 O.sub.3 with the accompanying formation of hematite bonds which accelerate the induration of the pellets in the subsequent firing process. If the indurating process proceeds in the condition such that magnetite is not sufficiently oxidized, the internal structures of the fired pellets will be non-uniform. The hematite and magnetite will be mixed, which will result in a reduced pellet crush strength and attendant impaired qualities.
The oxidation of magnetite pellets are affected by temperature, flow rate of gas, oxygen concentration in gas, grain size of material, porosity of the pellets, and the like. In general, the oxidation reaction starts at a temperature of 400.degree.C, and the higher the temperature, the more rapidly the oxidation reaction will proceed. However, if the oxidation reaction is carried out too rapidly, then abrupt oxidation can occur only on the outer peripheral surfaces of the pellets, resulting in a sintered and highly dense pellet surface structure. This dense and hard surface will prevent gas from diffusing into the interior of the pellets, resulting in insufficient oxidation. In such a case, the pellet structure will consist of a dense, outer peripheral layer of hematite bonds, and an inner layer having residual magnetite therein. In other words, a so-called double structure will result, which further often exhibits concentrically extending cracks along the boundaries of the inner and outer layers. The pellets of such a double or concentric structure will be characterized by reduced crush strength, as well as poor reduction characteristics.